An electric power steering apparatus (EPS) for applying a steering auxiliary force (an assist force) to a steering mechanism of a vehicle by using a rotational force of a motor gives a drive force of the motor by a transmission mechanism such as gear or belt via a reduction machine, and gives a steering assist force to the steering shaft or the rack shaft. The conventional electric power steering apparatus performs a feedback control on a motor current in order to accurately a generate torque of the steering assist force. The feedback control is directed for adjusting a motor applied-voltage such that a difference between a current command value and a motor current detection value is small, and the adjustment of the motor applied-voltage is generally made by adjustment of duty of a pulse width modulation (PWM)-control.
A typical configuration of an electric power steering apparatus will be described with reference to FIG. 1, and a column shaft (steering shaft or steering wheel shaft) 2 of a steering wheel (handle) 1 is coupled to steered wheels 8L and 8R via reduction gears 3, universal joints 4a and 4b, a pinion rack mechanism 5, and tie rods 6a and 6b and further via hub units 7a and 7b. Further, the column shaft 2 is inserted with a torsion bar, and a torque sensor 10 for detecting a steering torque Th, and a steering angle sensor 14 for detecting a steering angle θ of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is coupled with the column shaft 2 via the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current to be supplied to the motor 20 by a voltage control value Vref by which the current command value is compensated for. The vehicle speed Vel can be received from a CAN (Controller Area Network) or the like.
As well, the steering angle sensor 14 is not essential, and may not be provided, and a steering angle can be obtained from a rotational sensor such as resolver coupled to the motor 20.
The control unit 30 is connected with a controller area network (CAN) 40 for exchanging various items of information of the vehicle, and can receive the vehicle speed Vel from the CAN 40. Further, the control unit 30 can be connected with a non-CAN 41 for making communication with other than the CAN 40 or exchanging analog/digital signals, radio waves or the like.
The control unit 30 is mainly configured of a CPU (Central Processing Unit) (including MPU (Micro Processor Unit) or MCU (Micro Controller Unit)), and its typical functions performed by a program inside the CPU are as shown in FIG. 2.
The functions and operations of the control unit 30 will be described with reference to FIG. 2, and the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 (or the CAN 40) are inputted into a current command value calculating section 31, and the current command value calculating section 31 calculates a current command value Iref1 with a parameter of the vehicle speed Vel by using an assist map. The upper limit of the calculated current command value Iref1 is limited at a current limiting section 33, and a limited current command value Iref2 is inputted into a subtracting section 34. The subtracting section 34 obtains a deviation Iref3 (=Iref2−Im) between the current command value Iref2 and a feed-back motor current Im, and the deviation Iref3 is processed with a PI (Proportional-Integral)-control or the like at a current control section 35. The voltage control value Vref obtained by the current control is inputted into a PWM-control section 36 to calculate duty, and the motor 20 is PWM-driven with the duty via an inverter 37. The motor current Im of the motor 20 is detected by a motor current detector 38, and is fed back to the subtracting section 34.
In comparison with a conventional hydraulic power steering apparatus, the electric power steering apparatus mounts the motor and the gears thereon, and therefore has a problem that it has a large friction and a steering wheel returnability is deteriorated after the turn at an intersection or the like. In order to improve the steering wheel returnability at the intersection, a steering wheel return control has been widely used on the basis of a steering angle by using a steering angle sensor as disclosed in Japanese Patent No. 3551147 (Patent Document 1). That is, FIG. 3 shows a schematic configuration of the apparatus described in Patent Document 1, in which there is provided a steering wheel return control section 32 for calculating a steering wheel return current HR based on the steering angle θ and a steering angle speed ω and the vehicle speed Vel. The calculated steering wheel return current HR is added to the current command value Iref1 at an adding section 32A, and a current command value Iref4 corrected by the steering wheel return current HR is inputted into the current limiting section 33. However, the apparatus disclosed in Patent Document 1 mounts the steering angle sensor thereon, which causes an increase in cost, and thus the steering wheel return control without the need of a steering angle sensor is desired.
In this connection, there has been proposed an electric power steering apparatus for controlling the steering wheel return by using a wheel speed (Japanese Patent No. 3525541 (Patent Document 2)). However, since the electric power steering apparatus described in Patent Document 2 performs the steering wheel return control based on the steering angle estimated from the right and left wheel speed signals, there is a problem that the steering angle is erroneously estimated in case of vehicle slip on a snowy road or the like and the steering wheel rotates in a driver-unintended direction.
Further, there has been known an electric power steering apparatus for, when the steering angle estimated from the right and left rear wheel speed signals and the steering angle sensor value are compared and then a difference therebetween is abnormal relative to its threshold, reducing the control output using the estimated steering angle (preventing an irregular output) (Japanese Patent No. 4671435 (Patent Document 3)). However, the apparatus described in Patent Document 3 needs a steering angle sensor, and has a problem that an increase in cost is caused.